Wells are generally drilled into subsurface rocks to access fluids, such as hydrocarbons, stored in subterranean formations. Drilling fluids (e.g., drilling muds) are used within wells for various reasons, such as to inhibit flow of formation fluids into the wells, to clean and cool drill bits, and to remove wellbore cuttings. Drilling mud can be circulated through a well by pumping the drilling mud from a mud tank at the surface down into a well through a drill string. The drilling mud can exit the drill string at the bottom of the well and then return up the well through the annular space between the drill string and the well walls.
The returning drilling mud may include wellbore cuttings, other debris, and formation fluid. Various equipment can be used to condition and evaluate the returning drilling mud, which may include analyzing formation fluid mixed with the drilling mud to reconstruct the geological succession of formations penetrated during drilling and to assess the types of fluids encountered in the drilled formations. For instance, gaseous formation fluid (e.g., gaseous hydrocarbons, carbon dioxide, and hydrogen sulfide) may be carried up the well by the drilling mud, extracted from the mud at the surface, and analyzed with a gas chromatograph or other device to determine the composition of the gas.